The use of implantable cardioverter defibrillator (ICD) systems as a medical therapy for persons with abnormal heart conditions or cardiac arrhythmias is well known. Initially, ICD systems were used only to resuscitate or defibrillate a heart which had stopped pumping because there was no organized heart beat. This type of arrhythmia, referred to as ventricular fibrillation (VF), is relatively simple to detect and is fatal if not corrected in a few minutes. The general approach in using ICD systems to treat ventricular fibrillation is to deliver a relatively large electrical defibrillation countershock to electrodes implanted about the heart in an attempt to restart the electrical activity of the heart. In existing ICD systems, the defibrillation electrical countershocks are in the range of 25 to 40 joules, and are generated by high voltage capacitors within the ICD system that are charged to approximately 600 to 750 volts by one or more internal batteries.
ICD systems are now being used to treat other types of abnormal heart conditions, such as the main pumping chambers of the heart beating too fast. This type of arrhythmia, referred to as ventricular tachycardia (VT) can be clinically divided into two subclasses. The first VT subclass is a low rate ventricular tachycardia where the heart is beating in the range of approximately 120 to about 180 beats per minute. While a low rate VT is not normal, the patient is not in immediate danger of dying because there is still a perfusing pulse that can pump blood to the body. The second VT subclass is a high rate ventricular tachycardia where the heart is beating in the range of approximately 180 to about 250 beats per minute. In contrast to low rate VT, a patient with a high rate VT is in imminent danger of death within the next several minutes due to a significantly diminished or absent perfusing pulse.
High rate VT, despite its severity and grim prognosis, is treated differently from ventricular fibrillation. This is because, unlike a VF arrhythmia where there is no organized electrical activity of the heart, a high rate VT arrhythmia still exhibits a fairly organized and synchronous electrical activity of the heart and often can be treated by delivering a synchronized "cardioversion" countershock of lower energy that is in the range of 1 to 5 joules. If this cardioversion countershock is unsuccessful, existing ICD systems immediately resort to the use of a defibrillation countershock due to the serious nature of the high rate VT arrhythmia.
Low rate VT is also characterized by a synchronized electrical activity of the heart, but a low rate VT is usually able to generate a perfusing pulse. As a result, it is important in treating a low rate VT to avoid subjecting the patient to an electrical cardioversion therapy that could convert the patient from an abnormal, but life sustaining arrhythmia, to an abnormal and terminal arrhythmia. Because a low rate VT is not immediately life-threatening, avoidance of shock pain is a major goal. Thus, the usual approach for low rate VT is to deliver bursts of overdrive pacing pulses that will pace the heart at a rate greater than the low rate tachycardia. This technique utilizes pacemaker level energies of approximately 10 to 50 microjoules per pulse for a burst duration of approximately 10 pulses per burst. If the first burst is unsuccessful and the patient remains in a low rate VT, subsequent bursts are reattempted.
An ICD system is more than just a pulse-generating device that is implanted in the patient. The pulse-generating ICD is a part of a collection of electrical and mechanical components of an entire ICD system. The parts of the system include; the ICD itself, leads, a test instrument, and a programmer among other things. The programmer is the physician's window to the operation and the programmable settings of the ICD. The programmer allows a physician, usually a highly trained ICD specialist, or electrophysiologist (EP), to enter numerous parameters related to the operation of the ICD for each individual patient.
Because of the unique requirements of each patient and the different kinds of therapies to be delivered, there are numerous parameters that need to be programmed into the ICD, such as fibrillation detection rate, high rate tachycardia detection rate, and rate cutoff for low rate ventricular tachycardia, just to name a few. Clinical physicians understand a patient's medical history, but are not necessarily familiar with all of the ramifications and combinations of all of the parameters that need to be programmed into an ICD. Additionally, the parameters, in many cases, are not given in rates that the physician is familiar with, such as beats per minute, but rather are given as intervals such as 333 ms. Therefore, the physician must translate familiar units into unfamiliar alternate units when programming an ICD, a process which can be time consuming and difficult.
More importantly, except for the highly experienced ICD implanter, the physician may not be aware of the critical importance of setting a detection rate accurately as opposed to setting it 10% higher or lower than it should be. Accurate settings in an ICD are critical in applying an appropriate therapy. Setting a defibrillation or cardioversion detection rate too low may cause an unrequired defibrillation or cardioversion countershock to the heart which is both very painful and potentially damaging to the heart. Setting a detection rate too high may cause the failure to timely apply a countershock to a fibrillating heart and thus may allow a patient to die.
It would be desirable to provide an ICD system capable of being automatically programmed by a physician familiar with a patient's medical history, yet who may not know each and every parameter setting for ICD programming.